1. Field of the Invention
The invention relates generally to frequency modulation techniques and more specifically, to repetitive frequency sweep or ramp generators.
2. Description of the Prior Art
In the prior art, swept oscillators are extant for various purposes. Among these purposes is the provision of a continuously variable (over each of a series of repetitive cycles) local oscillator signal for a panoramic surveillance receiver. Where such receivers are of the relatively sophisticated type, such as the so-called compressive receiver described, for example, in U.S. Pat. No. 2,882,395, there is the additional requirement for a high degree of linearity of the function df/dt. Still further, the satisfactory operation of the so-called compressive receiver, which uses a dispersive delay line demands a high degree of linearity of the frequency ramp which provides the local oscillator function of such a compressive receiver.
In the prior art, various approaches to the linearity problem have been taken. Where the controlled oscillator, which actually generates the frequency ramp, is one of the forms of known voltage controlled oscillators, there are inherent non-linearities. That is, the instantaneous oscillator frequency is not a linear function of the instantaneous control voltage over a useful range of oscillator frequencies. Accordingly, the generation of a highly linear control voltage as a function of time does not accomplish the objective of providing a linear frequency ramp. Various open-loop techniques are available for improving this situation, all of these involving tailoring or pre-shaping of the oscillator control voltage as a function of time. In this way, a compensating distortion is introduced. Various function generators, such as diode shaping networks, etc., are available to effect this intentional distortion of the control voltage, the degree of compensation obtainable (i.e., the granuality of the correction) being related to the number of points at which correction is effected, and therefore the amount of equipment devoted to it.
Quite obviously, the open loop approach cannot compensate for component aging, temperature effects, etc.
A closed-loop system based on application of the frequency ramp to a discriminator to generate a correction signal offers a possible approach to a closed-loop system, however, the linearity of the discriminator itself then becomes a factor in the problem.
Specific prior art examples of sweep and frequency ramp generators in U.S. Patents include Nos.: 3,144,623; 3,504,293; 3,621,450; 3,764,933; 3,221,266; 3,528,033; 3,699,448; 3,872,406; 3,382,460; 3,530,399; 3,723,898 and 3,93l,586.
The manner in which the unique combination of the present invention deals with the problems of the prior art will be evident as this description proceeds.